This invention is directed to a variable displacement pump of the type having a plurality of rotatable, axially oriented pistons guided by a non-rotatable, tiltable swash plate which enables adjustment of pump displacement. More particularly, this invention is directed to a zero displacement positioner incorporated with a servo mechanism for returning the swash plate to a central neutral position in the event of a failure in the pump system.
Reversible variable displacement pumps are frequently employed in the drive systems of vehicles to supply fluid pressure to hydrostatic motors for driving the vehicle wheels. The components of such a drive system are subject to failures which may occur suddenly and unexpectedly during the operation of the vehicle.
Some examples of such failures are: breakage of the mechanical linkage between the operator control and the servo mechanism controlling the pump; malfunctions in the pump itself or in the hydrostatic motor; a rupture in the hydraulic lines between the pump and motor; or a loss in pump charging pressure. When such failures occur, it is often necessary to immediately stop the discharge of pressure fluid from the pump in order to prevent further damage. Discharge should also be stopped to prevent the loss of large quantities of hydraulic fluid or the possible injury of the operator. Present pumps lack the ability to automatically shut down the pump when a failure such as that described above occurs. Therefore, the necessary immediate remedial action is not accomplished and the undesirable consequences indicated above frequently occur.
Variable displacement pumps of the type described which include servo mechanisms for their operation also require a pilot pressure system for operation. These pilot pressure systems utilize a lower pressure than that discharged by the pump. This results in an additional disadvantage since the servo piston must be relatively large to provide sufficient force to overcome the swivel torque exerted by the pump pistons.